What is the difference between density-dependent and density-independent causes of fluctuations of the size of mammalian populations?

Irregular fluctuations are commonly produced by variations in climate, such as unusually hot, cold, dry, or natural catastrophes-such as fires, hailstorms, and hurricanes. These are density-independent factors because they affect a population whether it is crowded or dispersed. However, the spectacular fluctuations are density-dependent; that is they correlate with population crowding.